Varifocal lens assembly for still camera photography



N Du MU D. UH PU m S R 10 S o 4 O 3 p O F. G. BACK Dec. 26, 1961VARIFOCAL LENS ASSEMBLY FOR STILL CAMERA PHOTOGRAPHY X .z a 3 1 FiledFeb. 19, 1959 NdE - mmvron Frank G Back ATTORNEY I II H II I II h uUnited States Patent 3,014,406 VARIFOCAL LENS ASSEMBLY FOR STILL CAMERAPHOTOGRAPHY Frank G. Back, 55 Sea Cliff Ave., Glen Cove, N.Y. Filed Feb.19, 1959, Ser. No. 794,323 2 Claims. (CI. 88-57) This invention relatesto varifocal lens structures and particularly to a varifocal lensassembly for use in a single lens reflex still camera. This applicationis a continuation-in-part of my co-pending application Serial 745,009,filed June 27, 195 8, for Varifocal Lens Assembly, now US. Patent No.2,913,957.

Where it is desired to use varifocal lenses employing the principle ofimage shift compensation in the field of still camera photography,certain problems arise brought about by the fact that still camerasrequire as much as a 60 angle of field coverage whereas television andmotion picture photography rarely employ more than a 35 angle. Therequired image correction over the larger field in a still picture hasto be greater than that in television and motion picture photography.

Accordingly, it is an object of the present invention to produce thehigh quality image necessary in still picture photography despite thelarge relative aperture and a large field angle, while at the same timeutilizing the advantages of a varifocal lens.

Another object of the present invention is to provide a varifocal lenssystem having an image correction over the entire field of lighttransmission far superior to previously known devices.

A further object of the present invention is to provide a varifocal lensand relay for use with still cameras which will avoid the shortcomingsof presently known varifocal lenses which are not adapted to stillpicture photography.

A feature of the present invention is its varifocal lens assemblywhereby aberrations are kept constant over the entire range of the lenssection in front of the iris stop.

A further feature of the present invention is its use of a relay tocompensate for the constant aberrations in the varifocal portion of theassembly.

Still another feature of the present invention is its use of a relaywhich has a large relative aperture.

A further feature of the present invention is its distribution of therefractive powers within the relay in such manner as to minimizeresidual distortion.

A feature of the present invention is its use of varifocal lens systemsin which selected components consist of two or more elements separatedby small fixed lenslike air spaces wherein all of the surfaces facingsaid fixed air spaces are stronger than those facing the variable airspaces.

Still another feature of the present invention is the use of a varifocallens system in which certain lens surfaces which are concave towards theaperture stop must be stronger than those which are convex towards theaperture stop. 5

-A feature of the present invention is its use of asymmetric powerdistribution around the fixed small air spaces.

Th invention consists of the construction, combination and arrangementof parts, as herein illustrated, described and claimed.

In the accompanying drawings, forming a part hereof 3,014,406 PatentedDec. 26, 1961 are illustrated two forms of embodiment of the inventionand in which:

FIGURE 1 is a view in longitudinal section of complete opticallycompensated varifocal lens constituting one embodiment of the presentinvention with the movable element in the rear position indicated bydashed lines.

FIGURE 2 is a view in longitudinal section of a second form of relay foruse in conjunction with the varifocal lens system shown in FIGURE 1.

Referring to the drawing and specifically to FIGURE 1, 5 indicates afront lens member of dispersive power. A pair of axially slidable lensesfixed with respect to each other of collective power, hereinafterreferred to as the variator 6, is disposed behind the front lens 5. Astationary dispersive element 8 is disposed behind the variator 6 and isknown as the erector. The erector 8 also consists of a pair of lenses19, 20. A movable collective system hereinafter referred to as thecompensator 7 is linked to the variator 6 by means of a coupling member9. The compensator 7 has a strong positive power.

A stationary aperture stop 10 is located behind the compensator 7.Behind the aperture stop 10, there is disposed a multiple element system11, known as the re lay. The relay 11 has collective power in order toproduce a real image and is followed at the proper distance by the imageplane 21.

In the embodiments shown herein the small fixed air spaces areconsidered as having the properties of lenses bounded by the curvedsurfaces of the lens elements adjacent thereto. The variator 6 whichconsists of a pair of spaced lenses. 17 and 18, and which has beendescribed as being of collective power is separated by a small fixed airspace S which is also of collective power. The erector 8 consisting of apair of lenses 19, 20 has strong refractive surfaces R R which areseparated by a small fixed air space S which in contrast with thevariator S is strongly dispersive.

In order to produce the high quality image required by present day stillphotography at a large relative aperture and a large field angle,certain characteristics within the lens system must be present. Thedispersive front lens 5 must be shaped in such manner that its strongerdispersive surface R is concave toward the aperture stop 10. Inaddition, the movable variator 6 behind the front lens 5 and also thestationary erector 8 behind the variator 6, both of which consist of apair of lenses 17, 18, and 19, 20, in juxtaposition, and whose fixedinner spaces 8;, 8;, are enclosed by two strongly refractive surfaces RR and R R respectively, have said enclosing surfaces asymmetricallyarranged in such a way that the surfaces R R concave toward the aperturestop 10, is in each case, stronger than surfaces R R,, which are convextowards the aperture stop. The compensator 7, which is axially slidablewith the variator 6, is positioned between the erector 8 andthe aperturestop 10, has at least one collective outside surface R with a power ofmore than 25% and not more than of the total power of the varifocalobjective image in its middle position.

This relationship may'be expressed as follows:

3 Where =the power of the whole system in its middle position R=therefractive surface power It will be seen from the above that thefollowing relationships apply:

5 0.65i |4 Variator[ l.30 l I have found that the asymmetry of powerdistribution 90q, 1 between the strongly refractive surfaces R R R R9,().65 I l scompensatorl 1.30 1 adjacent to the fixed inner air gaps haveto be the larger, the faster the required speed of the lens (the greaterIf the basic power of each of the basic components is kept the diameterof the bundle of light rays entering the 10 within the above givenlimits, not only satisfactory comsystem) and the wider the desiredmaximum field angle. pensation for zoom deviation is achieved but alsothe zonal Accordingly, the inner surface R of the erector 8, whichresidual aberrations can be kept within very small limits. is concavetowards the aperture stop 10, must be more In the embodiment hereinillustrated in FIGURES 1 than stronger than but not more than 115% ofthe and 2, it is possible to achieve a varifocal objective withneighboring surface R enclosing the air gap S which 15 wide maximumfield angle in combination with a large surface is convex towards thestop 10. This means: relative aperture, so that the relay itself alsohas to have a large relative aperture. Varifocal systems made in aco a ocordance with the present invention with a large field angle and arelative aperture of up to 1:2.7, can be constructed, In the movableVaflatcl' the Inner Surface s which if in addition to the previouslymentioned characteristics, is concave toward the stop 10, must have morethan 40% v the relay 1 is made in the f ll i manner; but not more than220% greater refractive power than its The relay 11 disposed behind theaperture stop 0 has PP Surface 4 Which is c011vex towards the stop onthe side closest to the aperture stop 10, a lens element Thlsfelflhchshlp y 9150 he expressed: 12, which has a highly dispersiveconcave surface R toward the stop 10. The power of said surface R ispreferably more than 75% of but not more than 275% Because it isessential to the satisfactory functioning of of the Power of the wholesystem m Its Huddle posmon' the present lens systems that the asymmetrybe the greater the longer the light path from these neighboring surfaces0'75Pm |R15| 2'75m t0 the aperture 10 heccmes- Behind the lens 12 areasymmetric collective components I have also discovered that forObtaining y large 13, 14, 15, which in turn are followed by a meniscus16, field angles the pair of inner surfaces R R of the variaconcavetowards the image plane 21 Which are relatively f c h p 10, P In thisform of the relay 11, the strongly dispersive sur- Y have the'fcllcwlhgPower dlsmbutlon: face R close to the aperture stop 10, produces an R l66 R astigmatic field flattening, and reduces coma. The con- 5 4 cavesurface R of the meniscus 16, closest to the image The Surface R5 of thevariator 6 which is concave plane 21, serves to control distortion,particularly in view Wards the aperture stop 10 th f must have a Powelofthe large field angle. If the residual distortion has to greater than5/3 of the power of the adjoining surface 40 be kept 'Y small, It cannormally be achleved y at the R which is convex towards the stop.

An example of an optically compensated varifocal lens system made inaccordance with the foregoing is:

expense of coma. However, in the present invention, this result can alsobe avoided if the refractive power within the relay 11, is distributedas hereinafter set forth.

Radius (R) Thickness (d) and Glass Lens No. (mm) Air Spacing (S) (Cat.Index (N Dispersion Rel.) (V) R =+817.00 Front Lens 5 dl=2.50 EDF N=1.689 V =30.9

S|=6.00 to 26.00 R; =+372.20 Variator No. 1,17 d1==6.00 DBO N =1.620 V=60.0

81-1130 R; =+56.70 Varlator No. 2, 18 ds=8.00 DBO Nat-1.620 V:=60.0

S:2=2.00 to 2.00 R1 -Inf. Erector No. 1, 19 dt=2.00 LaO Nm=1.720 V4=50.3

S=4.25 R: 93.50 Erector N0. 2, 20 dt=1.80 LaC Nns=1-720 Vs= .3

R1o=+25.00 i

d=3.70 EDF Nm=1.721 V=29.3 Ru=+140.20 R St=1.80 to 21.80

u= 107. Oompensator 7 R +25 00 d1-2.00 DBG Nor-1.620 V1=60.0

(iv-51X! DF Dim-1.621 Vs=36.2 B t-+5835 St=21.00 to 1.00

Stop S7=4.50

Radius, thickness and spacing in millimeters.

An example of a relay for an optically compensated varifocal lens systemmade in accordance with the foregoing is:

6 In the relay 27 the performance of the lens system in the wide angleposition has been improved at the expense of the telescopic position.The relay 27, in other words,

(mm) Radius Thickness Glass Dis ersion Lens No. (R) (4), Air (Cat. IndexN n V') Spacing (8) Ref.)

12 Mk2) a 1 50 L c N 1 120 n I a RIP-327.40 nu V11 503 Sr =8.75 1sRIP-6M0 d 3 1o DBO N R": 28 00 i on n 603 So =0.50 14 Right d a DBO N isRip 63.10 11 m4 V11 600 Sro=0.50 15 Emil-5M0 a 3 to DB0 N 1 Ru 23L) 1 ms600 S11=13.50 1e REF-H3830 a s on DB0 N 1 s R:4=l67.80 0 d =1.50 EDFN1m=1.721 Vis=29.3 Rzs=+24.30

Equivalent focal length of whole system (E.F.L.) =80.0040.00.

Back focal length of whole system (B.F.L.) =41.10.

Radius, thickness and spacing in millimeters.

The power in the above relay is distributed in the following way:

The relay 11 consists of a plurality of components disposed in suchmanner that a negative lens 12 is located close to the aperture stop 10,having a hollow stronger refractive surface R toward the stop 10, whileon the image side there is a negative meniscus 16, concave towards theimage 21. Between said components there is a plurality of insidecomponents 13, 14, 15, which include two collective air spaces, suchthat the sum of their refractive power is more than twice but not morethan six times the equivalent power of the whole system in its middleposition.

This relationship may be expressed:

Referring to FIGURE 2 there is shown a second form of relay 27 for usewith the varifocal system shown in FIGURE 1. It will be seen that therelay 27 is disposed behind the aperture stop 10. On the side closest tothe aperture stop 10 there is provided at least two lens elements, oneof'which has a highly dispersive concave surface indicated at R towardsthe said stop 10. The absolute numerical power of the said surface R ismore than 75% but not more than 275% of the power of the whole system inits middle position. The lens 23 of the relay 27 is preceded by ameniscus 22, having very weak power. Behind the lenses 22, 23, is anasymmetric collec tive two lens component 24, 25, which in turn isfollowed by a meniscus 26 concave towards the image 21. In this secondembodiment as in the first, the strongly dispersive concave surface Rclose to the aperture stop 10 serves to bring about an astigmatic fieldflattening, and to reduce the previously mentioned coma. In the meniscus26, closest to the image plane 21, the concave surface R acts to controldistortion, which might be expected in view of the large field angle. Itwill be observed that the relay 27 consists of a plurality of componentsso arranged that close to the aperture stop 10 there is disposed anegative lens 23 with a hollow stronger refractive surface R towards thestop, while on the image side there is a negative meniscus 26 concavetowards the image 21. Between the components 23 and 26, there is aplurality of inside components 24 and 25, which include two smalllenslike air spaces S and S so arranged that the total power of thesetwo air spaces is greater than two and one-half times but not more thanseven and one-half times the equivalent power of the total varifocalobjective in its middle position if set for the geometric means ofmagnification.

shifts the image quality from the telescopic to the wide angle position.The use of the meniscus 22 shortens the assembly and permits a decreasein the diameter of all of the lenses and especially the two asymmetriccomponents 24, 25, in order to achieve this result without loss of theimage quality, the meniscus 22 must be of the following power: It musthave a power which lies between 0 and 0.75 times the absolute value ofthe power of the surface R on the lens 23, which means:

The following table gives the optical characteristics of till: relaysystem 27 shown in FIGURE 2 and described a ove:

Lens (mm.) Radius Thickness Glass Index Disper- No. (R) (d), Arr (Cat.Ref.) N D sion (V) Spacing (S) 22 Rle=+23.57 d 3 00 F Sn=3.30 23 R2s=-24.88 d 1 50 B rs= aSFS-l 1.7015 41.0

Rza==+122.00

S1s=2.50 R:o=80.70 24 dl1=3.20 LaK-17 1. 78847 50. 45

S14=0.50 Rtz=+600.00 25-.." dia=2.80 LaK-17 1. 78847 50. 45

Raa=54.32

S15=0.30 R34=+25.24 26.. dn=5.20 SK-16 1.6204 60. 29

d =2.00 F-2 1. 620 36. 34 R1s=+18.56

BaSFS stands for Special Dense Barium Flint (Schott). Equivalent focallength (E.F.L.)=fz=&7.00 to 40.00 (approih) oi the whole system.

Baclr focal length of whole system (B.F. L.) =40.10. Radius (R),thickness (d), and spacing (S) in millimeters.

In the present embodiment of the invention the stationary lenses in thevarifocal portion of the system have negative power and the movableelements are positive. However, it is known that a varifocal lens can bemade wherein the stationary lenses have positive power and the movablelenses are negative and such substitution can be made without departingfrom the spirit of the present invention. Notwithstanding such changesin the specific arrangement of lens elements, I have found that certainrelationships must be maintained in order to achieve the desiredresults. Some of the components have to consist of two or more lensesenclosing small fixed lens-like air spaces. Lens surfaces facing saidfixed air spaces within the system must be stronger than those facingthe variable air spaces. In addition of those lens surfaces adjacentsaid fixed air spaces, those which are concave towards the aperture stopmust be stronger than those surfaces which are convex towards theaperture stop.

The chromatically corrective cemented surfaces R in the erector 8, R inthe compensator R in the relay lens 16, and R in the relays lens 26, areall preferably disposed concave toward the aperture stop to achieve goodcolor correction as well as the chromatic variations of themonochromatic aberrations, especially spherical aberrations.

Having thus fully described the invention, what is claimed as new anddesired to be secured by Letters Patent of the United States, is:

1. A varifocal lens system for single lens reflex cameras comprising, afront section of variable focal length, including an axially slidableair spaced variator and compensator and a stationary erectortherebetween, an aperture stop spaced from the front section and a rearsection of fixed focal length including a relay having five elementstherein with optical characteristics of the following order wherein N isthe refractive index for the D line, V is Abbes dispersion number, LaKstands for lanthanum crown, BaSFS stands for special dense barium flint(Schott), F indicates flint and SK designates dense crown.

Radius, thickness and spacing in millimeters; (R to R )=the radius 2. AVarifocal lens assembly for use in still picture photography comprising,a front lens of dispersive power, a variator consisting of spacedaxially slidable lenses of collective power behind the front lens, afixed air space of strongly collective power between the variatorlenses, an erector behind the variator comprising spaced lenses, a fixedstrongly dispersive air space defined by the inner stronger refractivesurfaces of the erector, a compensator consisting of a second axiallyslidable collective lens system, said compensator being coupled to thevariator for axial movement therewith, a stationary aperture stop behindthe compensator and a relay of collective power hehind the stationarystop to produce an image upon an image plane, said relay having a firstpositive component adjacent the aperture stop of a power m of betweenzero and 0.75 times the absolute value of the power of the surface R28of the lens 23 of the relay system to fulfill the following condition: OO.75]R I said positive component being followed by a first negativecomponent, a highly dispersive front surface R on the first negativecomponent concave toward the aperture stop the absolute numerical powerof which is more than but less than 275% of the power of the wholesystem in its middle position, said front surface being adapted toeifect astigmatic field flattening and reduce coma, a plurality ofpositive components following the first negative component, two smalllens-like air spaces S14, S between said positive components the powerof said air spaces being greater than 2 /2 times but less than 7 /2times the equivalent power of the total Varifocal lens assembly in itsmiddle position to fulfill the mathematical condition: 2.51 (S +S 7.5@ alast negative component forming with the last of the plurality ofpositive components, a negative meniscus concave toward the image theconcave rear surface of which is adapted to control distortion caused bythe large field angle.

References Cited in the file of this patent UNITED STATES PATENTS1,584,272 Bertele May 11, 1926 2,566,485 Cuvillier Sept. 4, 19512,685,229 Schulz et a1. Aug. 3, 1954 2,704,487 Rosier Mar. 22, 19552,718,817 Back et al Sept. 27, 1955 2,719,457 Tripp Oct. 4, 19552,741,947 Back Apr. 17, 1956 2,778,272 Reymond Jan. 22, 1957 2,782,684Hopkins Feb. 26, 1957 FOREIGN PATENTS 1,150,483 France Aug. 12, 1957OTHER REFERENCES General Theory of Optically Compensated VarifocalSystems," Bergstein, Journal of the Optical Society of America, vol. 48,No. 3 (March 1958), p. 170.

